Automobile exhaust manifolds, mufflers, and other exhaust system parts are required to have high temperature strength and oxidation resistance. Therefore, ferritic stainless steel superior in heat resistance is being used. These parts are produced by press working steel sheet, so press formability of the steel sheet material is sought. On the other hand, the temperatures of the usage environments have been rising each year. It has become necessary to increase the amounts of Cr, Mo, Nb, and other alloying elements added so as to increase the high temperature strength, oxidation resistance, heat fatigue characteristics, etc. If the elements added increase, the workability of the steel sheet material ends up falling with simple production methods, therefore sometimes press forming was not possible.
Indicators of workability include indicators of the ductility, deep drawability, etc. In working the above exhaust parts, the basic indicators of the elongation and r value become important. For improvement of the r value, increasing the cold rolling reduction rate is effective, but since the above parts use relatively thick materials (1.5 to 2 mm or so) as materials, the cold rolling reduction rate cannot be sufficiently secured in current production processes where the thickness of the cold rolling material is limited to a certain extent.
To solve this problem, means have been taken with regard to the ingredients or method of production for improving the r value without damaging the high temperature characteristics.
In the past, to improve the shapeability of the ferritic stainless steel sheet used as the above heat resistant steel, adjustment of the composition has been disclosed as shown in Japanese Patent Publication No. 9-279312, but with this alone, there was the problem of press cracking in thick materials with relatively low cold rolling reduction rates.
Japanese Patent Publication No. 2002-30346 prescribes the optimal hot rolled sheet annealing temperature from the relationship between the hot rolling finishing start temperature and end temperature and Nb content and the hot rolled sheet annealing temperature, but due to the effect of other elements (C, N, Cr, Mo, etc.) involved in Nb-based precipitates, sufficient workability sometimes cannot be obtained by this alone. Further, Japanese Patent Publication No. 8-199235 discloses a method of aging a hot rolled sheet in the range of 650 to 900° C. for 1 to 30 hours. The technical idea is to cause the Nb-based precipitates to precipitate before cold rolling so as to promote recrystallization, but with this method as well, sometimes sufficient workability cannot be obtained and the productivity remarkably falls. In general, hot rolled steel sheet is coiled for supply to the next process, but when aged in the coil state, it is learned that the variation of the structure and the workability when made into the final product become remarkable in the longitudinal direction of the coil (outermost coiled part and innermost coiled part).